1. Field of the Invention
The invention elates to the surface modification of fluoropolymers for the purpose of facilitating adhesion and bondability, and more particularly to fluoro-oxidation of ethylene-chlorotrifluoroethylene copolymer (ECTFE).
2. Description of the Related Art
Compared to other polymers, fluoropolymers have outstanding resistance to chemical attack and remain stable at high temperature. Therefore they would appear to be outstanding candidate materials for chemical barrier coatings. But the physical properties that provide resistance and stability, viz., low surface-energy, very long chains, chemical inertness, high molecular weight, high melt viscosity with a narrow temperature range between melt and degradation, and poor heat conductivity, make them very difficult to adhere and bond to other materials.
ECTFE, a melt-processable fluoropolymer with a 1:1 alternating copolymer structure of ethylene and chlorotrifluoroethylene, is particularly suitable for chemical barrier coatings. ECTFE has been shown to provide excellent chemical and abrasion resistance, extremely low permeability to liquids, gases and vapors, a low dielectric constant, a broad range of useful temperature between cryogenic and 300xc2x0 F. (149xc2x0 C.), and low flame spread and smoke generation. ECTFE also is the toughest fluoropolymer and offers excellent chemical resistance to a wide variety of corrosive chemicals and organic solvents, as well as to strong acids, chlorine, and aqueous caustics. No known solvent dissolves or stress cracks ECTFE at temperatures below 250xc2x0 F. (120xc2x0 C.).
ECTFE is manufactured as HALAR(trademark) pellets by the Ausimont USA plant in Orange, Tex. The pellet form can then be converted into powder, solids, sheeting, or extruded film. Although ECTFE can be machined, welded and thermoformed, adhering the film to polymeric and metallic surfaces and adhering three-dimensional ECTFE objects to three-dimensional polymeric and metallic objects have heretofore not been achieved. Where used as lining material for the interior of piping, either a powder coating is electrostatically applied to the pipe interior surface or the pipe is lined with tubular sheeting. Kem-Tuff(trademark) process exhaust systems manufactured by GDS Manufacturing of Williston, Vermont have a primer and ECTFE top coat electrostatically applied to a stainless steel substrate. Electro Chemical Engineering and Manufacturing Co. of Emmaus, Pennsylvania manufactures Duro-Bond(trademark) sheet linings consisting of a layer of ECTFE laminated to a fabric or vulcanized soft rubber backing. Lining Technologies, Inc. of Denham Springs, Louisiana manufactures ECTFE linings for tankage and piping both in powder-applied and sheeting form.
Techniques for bonding fluoropolymers to otherwise incompatible materials are described in the related art. U.S. Pat. No. 5,460,661 to W. C. Maynard, Jr. discloses a method for bonding a fluoropolymer to the surface of a metal substrate. The fluoropolymer is applied to the surface as a powder and then heated above its transition temperature (i.e., the temperature at which the melted fluoropolymer flows together or otherwise transitions from a molten, non-agglomerated state to a continuous molten state), causing it to flow out and thereby form a unitary, void-free coating. The fluoropolymer-coated substrate is then held at a temperature above the melting temperature but below the transition temperature for a sufficient time to allow the halogen (the fluorine or chlorine from the halogen-polymer subunits in the fluoropolymer) to chemically bond to the metal. U.S. Pat. No. 4,865,711 to W. C. Kittler discloses a method for treating the surface of polymers of low surface energy, such as fluoropolymers. A thin layer of carbon is deposited on the surface of a fluoropolymer, polyimide, polyester, or polyolefin. Deposition is preferably carried out by sputtering, producing a carbon layer less than 300 angstroms (xc3x85) in depth. Polymeric material so treated may find use as layers in laminates and as substrates for deposition of metals. U.S. Pat. No. 4,886,689 to A. M. Kotliar et al. discloses a method for bonding fluoropolymers to polyolefins using an adhesive composed of mechanically interlocked bonds, i.e., bonds where molten polymer fractions entwine and wrap around one another in the melt, and are solidified in this state. The adhesive is a melt blend of the polymers contained in the layers it is desired to laminate. Alternatively, the adhesive may contain polymers that are not identical to those in the layers but which are sufficiently similar to a corresponding component in the layers. U.S. Pat. No. 5,093,403 to S. E. Rau et al. discloses a method for bonding a fluoropolymer to a metal substrate wherein a resinous coating is formed by fusing a composition including a major amount of the fluoropolymer and a minor amount of one or more additives which improve(s) properties such as corrosion-resistance, abrasion-resistance, and bonding characteristics. The compositions fall into three classes based upon the uses to which they are put: primer coatings, barrier coatings, and abrasion/wear-resistant functional coatings. Primer coatings bond very strongly to the underlying metal substrate and themselves provide a substrate to which coatings having other properties may be bonded. Some compositions bond strongly to a metal substrate and also provide a barrier to chemical attack. Compositions which bond most strongly to a substrate are useful also as abrasion-resistant coatings applied directly to the substrate or over other polymer coatings. U.S. Pat. No. 5,152,323 to D. A. Shotts et al. discloses a metal pipe with an inner thermoplastic sleeve melt-bonded to the pipe. The sleeve material may be a fluoropolymer such as ECTFE or any other type of melt-processable thermoplastic.
U.S. Pat. No. 4,822,426 discloses a family of primer compositions which allow bonding together highly crystalline polymeric resin substrates and painting or printing on such substrates. A composition consists essentially of: (a) at least one member selected from the group consisting of organometallic compounds, natural resins, and synthetic resins; and (b) at least one fluorine-containing compound which is a linear or cyclic hydrocarbon having a polar group such as a hydroxyl or carbonyl group in the molecule in which part or all of the hydrogen atoms are substituted with fluorine atoms.
Modification of plastic surfaces to improve their surface properties is described in the related art. U.S. Pat. No. 5,948,484 to Y. Gudimenko et al. discloses a process for modifying substrate surfaces which provides improved resistance to erosion, decreased permeability to water vapor and oxygen, control of hydrophobicity, and in some cases changes in properties such as friction coefficient, surface resistivity, ultraviolet/visible/infrared transmissivity, and adhesion. The process includes: surface activation of the substrate wherein reactive hydrogen groups are formed in a surface region; and silylation of at least a portion of the reactive hydrogen groups with a silylating agent, whereby silicon-containing groups of the silylating agent become incorporated in the surface region. Preferably, surface activation occurs when the substrate is exposed to a combination of ultraviolet radiation and oxygen, thus photo-oxidizing the substrate. After surface activation, the reactive hydrogen groups in the surface region are reacted with a silylating agent, replacing the hydrogen atoms of the active hydrogen groups with silicon-containing groups. The process preferably includes a third, oxidative stabilization step wherein silcon-containing polymer chains in the surface region are converted into stable structures enriched with silicon and oxygen. U.S. Pat. No. 5,098,618 to J. Zelez discloses a process for increasing hydrophilic wettability wherein a plastic substrate is exposed to ultraviolet radiation in the presence of monatomic oxygen for about 5 to 60 minutes. Suitable substrates include polyethylene, polypropylene, polystyrene, polymethylmethacrylate, silcone-elastomers, polyurethanes, polyamides, polyimides and polyethers.
Fluoro-oxidation of polymers is described in the related art. U.S. Pat. No. 4,020,223 to D. D. Dixon et al. discloses a method for surface treating fiber form synthetic polyolefin or polyacrylonitrile resins using low levels of monatomic fluorine in combination with low levels of monatomic oxygen and an inert carrier gas. A fluorinated carboxylated layer formed on the polymer surface results in enhanced water transport and soil release characteristics. U.S. Pat. No. 5,073,175 to M. Anand et al. discloses a process for fluoro-oxidizing polymeric gas separation membranes which have improved gas separation selectivity for gas mixtures such as O2/N2. Fluoro-oxidation is carried out by contacting the polymer surface either simultaneously or sequentially with a reactive fluorine source and an oxidation source.
None of these references addresses adhering or bonding ECTFE film to polymeric or metallic substrates. To be commercially viable in the manufacture of chemically resistant ductwork the simplicity of implementing a process for modifying film surface characteristics to enable adhesion and the cost of film so modified should be competitive with the alternatives of ECTFE powder deposition and welded or mechanically attached tubular sheeting. Nor do any of these references address adhering or bonding three-dimensional objects fabricated from ECTFE to three-dimensional polymeric or metallic objects.
In view of the limitations of the related art, it is a primary object of the present invention to provide a process for modifying the surface properties of ECTFE film to enable adhesion to polymeric and metallic surfaces.
Another object of the invention is to provide surface-modified ECTFE film which can be adhesively bonded to polymeric and metallic surfaces with any of a wide range of liquid adhesives.
Yet another object of the invention is to provide a surface modification process and a surface-modified film which are cost-effective in manufacturing ECTFE linings for tankage, piping and ductwork.
Still another object of the invention is to provide surface-modified three-dimensional ECTFE objects which can be adhesively bonded to polymeric and metallic objects.
Other objects of the invention will become evident when the following description is considered.
These and other objects are achieved by the present invention which in a first aspect provides a process for modifying at least one surface of a fluoropolymer consisting essentially of a 1:1 alternating copolymer of ethylene and chlorotrifluoroethylene, i.e., ECTFE. Each surface is exposed to a gaseous composition including effective concentrations of molecular fluorine and molecular oxygen for a period of time sufficient to increase the surface energy of the surface, thereby sufficiently decreasing the surface contact angle to allow bonding between the surface and an adhesive.
In another aspect the invention provides a fluoropolymer film having at least one fluoro-oxidized surface. The film consists essentially of ECTFE wherein each such surface has a contact angle of less than about 50 degrees.
In still another aspect the invention provides a three-dimensional ECTFE object having at least one fluoro-oxidized surface wherein each such surface has a contact angle of less than about 50 degrees.